Electron gun



April 15, 1958 c. K. BIRDSALL ELECTRON GUN Filed Feb. 25, 1955 IN VEN TOR.

HTTOE/VEH United States PatentO ELECTRON GUN Charles K. Birdsall, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calili, a corporation of Delaware Application February 25, 1955, Serial No. 490,607 7 Claims. (Cl. 315-3) This invention relates to microwave tubes and more particularly to an electron gun for producing a cylindrical electron stream suitable for use in beam type aruplifiers such as traveling-wave tubes.

In order to form a cylindrical electron stream of the electrons emitted at the end of a flat or concave cathode, it is at present the practice to employ a frusto-conical electrode and dish-shaped anodes having registering aper tures at their centers. The anodes are then maintained at potentials which increase stepwise with their distance from the cathode. In order to produce a perfect cylindrical electron flow it would be most desirable to have an infinite number of anodes spaced an infinitesimal distance apart. Ideally, the anodes would then have continuously increasing positive potentials proportional to their distance from the cathode. In actual practice different anodes must in fact be spaced a finite distance apart and be maintained at distinctly different potentials and usually there is but one anode. Furthermore, the apertured frusto-conical focusing electrode or the dishshaped anodes of the prior art must necessarily extend radially to a diameter substantially larger than that of the stream. For this reason the end of a traveling-Wave tube envelope housing the gun is generally considerably larger in diameterthan any other portion of the tube. The fact that the diameter of the envelope needs to be much larger than that of the stream in turn forces the inside diameter of the focusing solenoid normally employed to focus the electron stream of the tube to be unduly large and thus consume much more power than would otherwise be necessary.

It is therefore an object of the invention to provide an improved electron gun for producing a cylindrical electron stream.

It is another object of the invention to provide an electron gun having an unusually small diameter for producing a cylindrical electron stream.

In accordance with the invention a cathode having a circular electron emissive surface is employed as an electron source. A hollow ceramic cylinder is then positioned adjacent the electron emissive surface of the cathode. The cylinder is coated internally with a resistive material across which a potential is applied in a manner to provide a .potential variation which is substantially proportional to the four-thirds power of the distance from the cathode when the electron flow is to be parallel to the axis. Further, the electric field is directed along the desired path of flow of the electrons. Electrons emitted at the electron emissive surface of the cathode are thus directed along linear parallel paths to form a cylindrical electron stream which may be solid or hollow depending on the configuration of the cathode.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a traveling-wave tube employing an embodiment of the electron gun of the present invention;

Fig. 2 is an enlarged sectional view of the electron gun of the present invention shown in Fig. 1; and i Fig. 3 is a sectional view of another embodiment of the electron gun of the present invention.

Referring to the drawing, an embodiment of the electron gun of the present invention is shown in Fig. l. A traveling-wave tube 10 in which the gun of the pres ent invention may be employed with advantage is also shown in Fig. 1, including an input matching cavity 12 having a coaxial input cable 14 and an output matching cavity 116 with a coaxial output cable 18, and an evacuated envelope 20 which provides the evacuated chamber of the traveling-wave tube 10, consists of a long uniform cylinder.

At the left extremity of the envelope 20 an electron gun 22 is located for developing a cylindrical electron stream. The gun 22, which is more clearly illustrated in Fig. 2, comprises simply a cylindrical cathode 24 having a flat or button-shaped end portion and which is provided with a heater 26. A hollow cylindrical nonconductive form 28 is provided internally with a resistive coating 27-. The cathode 24 has a circular electron emissive surface covered with an electron emissive coating 2?. Heater 26 is supplied with direct current by means of a source of potential, such as battery 3%). Cathode 24 is referenced to a potential considerably below ground by a source of potential 32 having its positive terminal grounded while its negative terminal is connected to cathode 24 and to the negative terminal of battery 30. A voltage of the order of 1000 volts negative with respect to ground is representative of the voltage normally impressed upon cathode 24 by potential source 32. The end of the resistive coating 27 adjacent the cathode 24 is in electrical contact with cathode 24 and hence is connected to the negative terminal of potential source 32 while the opposite end of the resistive coating 27 is grounded. Non-conductive cylindrical form 28 may be made of any suitable dielectric or ceramic material which is insulating and resistive coating 27 may be made of any one of many commercial preparations such as the one known under the trade name of Aquadag. The resistive coating 27 may be applied by spraying or brushing a liquid suspension of the resistive material forming the resistive coating 27 onto the internal surface of the ceramic cylinder 28.

A potential distribution known as the Child-Langmuir distribution must be produced axially throughout the electron stream formed by the electrons emitted from the electron emissive material 29. The Child-Langmuir distribution requires that the axial potential distribution in the direction of electron flow be such that the potential of the stream varies proportionally for parallel elec' tron flow with the four-thirds power of distance from the cathode 24 and the electric field be directed parallel to the flow. It is obvious that in order to produce such a distribution with the resistive coating 27 the resistance of the resistive coating 27 must vary proportionally with the same four-thirds power of the distance from the cathode 24.

Referring again to Fig. l, a solenoid 54 is axially positioned symmetrically about the complete length of envelope 20. An appropriate direct-current is maintained in solenoid 54 by means of apotential source, such as a battery 56, so as to produce a magnetic field of the 3 order of 1000 gauss running axially along the length of the traveling-wave tube. The purpose of this magnetic field is to keep the electron stream focused or constrained along a path coextensive with and coaxial with the elongated portion of envelope after the stream has left the gun region.

Proceeding along from the electron gun 22 in the direction of flow of the electron stream, there are positioned successively about the path of the electron stream, a matching ferrule 58 connected by a lead 60 to a helix 62 which is, in turn, connected by a lead 64 to a matching ferrule 66. A collector 68 is positioned at the end of the path so as to intercept the stream electrons.

Helix 62 which serves as a slow-wave circuit for traveling-wave tube 10, preferably has aninner diameter substantially equal to the inner diameter of ferrules 58 and 66 so that the stream electrons can be made to pass as close to helix 62 as possible without being intercepted by the latter. A material such as tungsten is suitable for making helix 62, the principal requirement being that it retain its form, especially with respect to its pitch and diameter. In accordance with common practice the potential of helix 62 is maintained at ground by grounding ferrule 66.

As previously mentioned, helix 62 is connected to ferrules 58 and 66 by leads 60 and 64, respectively. Leads 60 and 64 are located parallel to the electric fields excited within matching cavities 12 and 16. Matching cavity 12 has the configuration of a rectangular toroid with a concentric collar 70 disposed about and spaced from matching ferrule 58. An opening 72 in the end plate of cavity 12 facing the left end of the helix 62 allows the full length of lead 60 to be energized and, in addition, decreases the tendency of the electric field produced by the potential on the cavity from disturbing the flow of electrons in the streams. Cavity 16 is similarly shaped, having a corresponding concentric collar 74 arranged about and spaced from matching ferrule 66 and an opening 76 facing the right end of helix 62.

The center conductor 78 of coaxial cable 14 extends through an aperture in the annular wall of cavity 12 and is connected to concentric collar '70 while the outer conductor of cable 14 is bonded to the periphery of the aperture. Likewise, the center conductor 86 of coaxial cable 18 extends through an aperture in the annular wall of cavity 16 and is connected to concentric collar 74 while the outer conductor of cable 18 is bonded to the periphery of the aperture in the same manner as before. Cavities 12 and 16 are fabricated with an inner surface composed of highly conductive material and are broadly resonant so as not to limit the frequency of operation. The configuration, shown and described for the cavities 12 and 16 in the drawing, provides suitable matching from helix 62 to coaxial cables 14 and 18 over a range of frequencies such as, for example, from 2000 to 4000 megacycles per second.

A potential of the order of 200 volts positive with respect to that applied to helix is applied to collector 68 in order to prevent secondary electrons which may be produced by the stream electrons impinging on its surface frcrn reaching helix 63 or ferrule 66. This potential is applied by means of a connection from collector 68 to the positive terminal of a source 84, the negative terminal of which is referenced to ground.

In the operation of amplifier 16, an input signal to be amplified is applied through input coaxial cable 14 to input cavity 12. The input wave in flowing along the exposed portion of conductor 82 within cavity 12 excites an electromagnetic field within the cavity. This electromagnetic field induces a corresponding voltage on lead 60 connecting ferrule 58 to helix 62 to launch a. traveling-wave along the helix 62. .Interaction between the main electron stream and the traveling-wave results in a transfer of energy from the stream to the wave causing it to grow" or increase in amplitude. The gain of 4 the wave is then proportional to the magnitude of the electron stream current.

At the end of helix 62, the amplified electromagnetic wave, in flowing along lead 64 connecting helix 62 to ferrule 66, excites an electric field in cavity 16. This electric field induces a corresponding signal in center conductor 84 of coaxial cable 18. This output signal may then be utilized.

An alternative embodiment of the electron gun of the present invention is shown in Fig. 3 comprising a cathode having an emission surface 132 concave to ward the left as viewed in the drawing. Cathode 1.30 is provided with the filament 26, connected to battery 30 and potential source 32. The emission surface 132 is appropriately coated with a layer 134 of an electron emissive material. A hollow conical ceramic form 136 is disposed adjacent cathode 130 with an internal surface coated with a resistive layer 1-38. An accelerating anode of suitable configuration is then disposed adjacent the ceramic form 136 and maintained at a potential somewhat higher than that of the right end of resistive layer 138 by means of a potential source 142.

The electron guns shown in Figs. 2 and 3 operate in an analogous manner. The gun 22 of Fig. 2 represents a simple planar gun whereas the gun of Fig. 3 represents a converging beam gun. The potential variation at the edge of an electron stream produced by a planar gun should be a four-thirds power of distance from the gun cathode. The ideal potential variation at the stream edge in a converging beam gun is a somewhat different function of distance. For relatively low-perveance guns, the ideal potential distribution is very nearly equal to the four-thirds power of distance. If a more accurate analysis is required of the ideal distribution, then the thickness of resistive layer 138 at any certain distance from cathode 130 may be determined by means of an electrolytic tank in a manner well known in the art.

By constructing an electron gun in accordance with the above teachings, a gun having an unusually small diameter may be obtained which may save a considerable portion of the power dissipated in a focusing solenoid, e. g. the solenoid 54 in Fig. l. The matching cavities 12 and 16 need not be as large in diameter as shown in Fig. 1 and, alternatively, thin rectangular waveguides may be substituted therefor.

What is claimed is:

1. An electron gun for producing a cylindrical electron stream, said gun comprising a cathode, a non-conductive hollow body having an internal surface converging away from said cathode, said hollow body being disposed about said stream, an anode comprising a resistive coating disposed on the internal surface of said body and being electrically coupled at the end nearest said cathode to said cathode, and a source of potential coupled between said cathode and the opposite end of said resistive coating.

2. An electron gun for producing a solid, cylindrical electron stream, said gun comprising a cathode having a substantially circular electron emissive surface, a nonconductive, hollow cylindrical body disposed about said stream, an anode comprising a resistive coating disposed on the internal surface of said body and being electrically coupled at the end nearest said cathode to said cathode, and a source of potential coupled between said cathode and the opposite end of said resistive coating.

3. The invention as defined in claim 2, wherein said resistive coating has a resistance increasing with the distance from the electron emissive surface of said cathode.

4. The invention as defined in claim 2, wherein said resistive coating has a resistance measured between said cathode and a point x on said resistive coating increasing at a rate substantially proportional to r where x is the distance from the electron emissive surface of said cathode.

5. An electron gun for producing a cylindrical electron stream, said gun comprising a cathode having a substantially circular electron emissive surface, a hollow ceramic body disposed about said stream and adjacent said cathode, a resistive coating disposed on the internal surface of said ceramic body, means for maintaining the end of said resistive coating adjacent said cathode at substantially the same potential as that of said cathode, and means for maintaining the opposite end of said resistive coating at a potential positive with respect to that of said cathode so as to provide an accelerating potential distribution along said hollow body increasing with axial distance from said cathode, whereby the electrons in said stream are continuously accelerated by the increasing potential field presented to them by said accelerating potential distribution along said hollow body within the electron gun.

6. An electron gun for producing a solid, cylindrical electron stream, said gun comprising a cathode maintained at a predetermined potential and having an electron emissive surface substantially concave in a direction opposite to that of the electron flow, a hollow ceramic cylinder disposed about said stream adjacent said cathode, a resistive coating disposed on the internal surface of said ceramic cylinder and being electrically connected to said cathode whereby the end of said coating adjacent said cathode is maintained at said predetermined potential, said resistive coating having a thickness variation to produce a collimated electron flow from said cathode, and means for maintaining the end of said resistive coating remote from said cathode at a potential positive with respect to the opposite end of said resistive coating so as to provide within the electron gun an ac- 6 celerating potential distribution along said hollow body increasing with axial distance from said cathode, whereby the electrons in said stream are continuously accelerated by the increasing potential field presented to them by said accelerating potential distribution along said hollow body.

7. An electron gun for producing a solid, cylindrical electron stream, said gun comprising a cathode having a substantially flat circular electron emissive surface, a hollow ceramic body disposed about said cathode and extending axially therefrom about said electron stream, a resistive coating disposed on the internal surface of said ceramic body electrically and physically connected to said cathode, said resistive coating having a thickness variation with respect to x, the distance from the electron emissive surface of said cathode, substantially proportional to x means for maintaining the end of said resistive coating remote from said cathode at a potential positive with respect to the opposite end of said resistive coating and cathode, so as to provide within the electron gun an accelerating potential distribution along said hollow body increasing with axial distance from said cathode, whereby the electrons in said stream are continuously accelerated by the increasing potential field presented to them by said accelerating potential distribution along said hollow body.

References Cited in the file of this patent UNITED STATES PATENTS 2,123,636 Schwartz July 12, 1938 2,626,371 Barnett et a1. Jan. 20, 1953 2,672,572 Tiley Mar. 16, 1954 

